Add EXISTS subquery support to Ballista

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
I want to support EXISTS subquery expression in Ballista but we need to refactor the ballista and datafusion proto files to allow LogicalExprNode to reference a LogicalPlanNode.

Describe the solution you'd like
As described above.

Describe alternatives you've considered
None

Additional context
None

[andygrove]

This is not needed now that we rewrite most subqueries as joins. We do not support other types of subquery.